Hi,
Has anyone replaced a heater core? I hear they're a pain to replace!! Is this something I need a professional to do?

I've worked as a mechanic before and I wouldn't/coudn't do it. It is a pretty big job. The entire dash comes out and the stuff underneath the dash too. It gets stripped all the way down to the firewall. If I had to do it over again I'd buy an electric heater and just plug off the heater core.